1. Field of the Invention
The present invention concerns in-flight sensors on board aircraft for detecting airborne liquid water droplets and ice crystals.
2. Background
An in-flight ice and icing conditions detection system should be able to distinguish between airborne liquid water droplets and ice crystals. Prior art approaches to distinguishing liquid water droplets from ice crystals measure the depolarization of the backscattered light from a polarized laser beam.
U.S. Pat. No. 7,370,525 is directed to a dual channel inflight ice detection system which detects ice accretion on an aircraft surface. The system illuminates an ice collecting surface mounted on an aircraft with linearly polarized light. The backscattered light is acquired in two light conductors, one with polarization sensitivity aligned to the transmitted light and the second with polarization sensitivity orthogonal to the first. The presence of ice is determined by the change in the ratio of light intensities in the two light conductors.
U.S. Pat. No. 6,819,265 is directed to an ice detection warning system mountable on-board an aircraft for in-flight monitoring of the airspace ahead of the aircraft. The system comprises: a first plurality of optical elements configured to direct a pulsed laser beam at a first wavelength from a laser source into the airspace ahead of the aircraft; a second plurality of optical elements configured to separate received backscattered light from the laser beam into a plurality of pre-determined wavelengths; a plurality of light detectors for detecting the light of the separated plurality of wavelengths, respectively, and generating respectively corresponding plurality of electrical signals representative of the light detected thereby; and a processor for processing the plurality of electrical signals to determine if airspace conditions ahead of the aircraft are likely to cause ice accretion on the surface of the aircraft, and for generating a warning indicative thereof.
U.S. Pat. No. 6,091,335 is directed to an optical device on board an aircraft for detecting icing conditions outside the boundary layer of the aircraft. The device, which also employs linearly polarized light, includes an optical beam emitter to create at least one illuminated external volume of measurement through which there circulates a flow of air charged with water particles. Also included is a collector optical system for the collection of at least a part of the light beam back-scattered by the water droplets (the external volume of measurement being located on the optical axis of the collector optical system). A photodetector is used to detect backscattered light, while a signal processor processes the signal delivered by the photodetector to compute the severity of the icing conditions. The collector optical system further comprises means for differentiating between liquid water droplets and ice crystals by analyzing the polarization of the light collected. Information is provided for the calculated severity of the icing conditions.
FIG. 1 shows a prior art optical device 100 made in accordance with the '335 patent. The device 100 includes an emission subsystem 106, a collector optical subsystem 108, a photodetection subsystem 124, and electronic circuitry 154. A power supply 130 provides the power needed for the emission subsystem 106 and the photodetection subsystem 124 and the electronic circuitry 154.
The emission subsystem 106 comprises a polarized laser diode 110 (e.g., emitting in the visible or near infra-red range (between 400 nm and 1500 nm)), a collimation optical unit 112 and a deflection prism 114, so as to create an illuminated volume of measurement V on the optical axis AA′ of the collector optical system, through the porthole window H of the aircraft, located on the skin P of the aircraft. Typically, the emission means can emit light outside the skin of the aircraft, at a distance of approximately 100 mm. This makes it possible to carry out measurements outside the boundary layer of the aircraft (the area in which measurements would not be representative) while preserving a reasonable level of emitted power, a volume of polarized light whose dimensions are smaller than one mm, namely a value below which it can be shown that the probability of having two particles simultaneously in the volume of measurement is almost zero.
The collector optical subsystem 108 comprises a focusing optical unit 116 with a small diameter (10 to 20 mm) and a narrow-band optical filter module 118 (working typically in the bandwidth range of about 10 nm) that enables the filtering of solar illumination. The collector optical system may also comprise a polarization separator 122, which may be either a polarizing separator cube or a birefringent separator.
The photodetection subsystem 124 comprises two photodetectors 126 and 128 which recover cross-polarized optical beams. The polarization of the optical beam backscattered by the liquid water droplets is identical to that of the incidental optical beam. A first signal I1L relating to this backscattered optical beam is recovered at a photodetector 126. On the other hand, the photodetector 128 recovers a second signal, which is real and relates to a cross polarization (I2L=0).
The polarization of the optical beam backscattered by the ice crystals is modified in relation to that of the incidental optical beam. The two photodetectors 126 and 128 recover signals I1S and I2S, respectively, when a particle passes into the volume of measurement. The output of the first photodetector 126 passes though a first filter 136 and then a first variable gain amplifier 146. Similarly, the output of the second photodetector is passed through a second filter 146 and then a second variable gain amplifier 148.
The electronic circuitry 154 carries out the mathematical processing of the received signals such as summation, comparison of the two channels, computation of the severity of the icing conditions, etc. The electronic circuitry 154 can compute the total volume of the droplets of water that pass per unit of time by summing the signals obtained on the two channels. It also computes the “de-polarization” by computing the ratio between the signals of the two channels and thus determines the state (i.e. liquid water versus ice water) of the particles.
U.S. Pat. No. 5,617,076 is directed to a system for detecting de-polarizing substance, such as ice or snow, on a surface which specularly reflects light. The '076 patent discloses that circularly polarized light may be used in conjunction with such a system.